I don't know what they are called and I have never seen a solo flickering LED. 

The link that TRM posted looks good, but it is a bit complicated.  

I would just take a flickering LED candle and hack it apart.   These candles are pretty cheap, and when you do take one apart, you will find that they use a very small circuit to do the flicker.

But those candles will not do what TRM would like, they will never be bright. 

And don't forget to search model railroad sites:

http://www.modeltrainsoftware.com/fire-led.html

http://www.building-your-model-railroad.com/fake-fire.html

http://www.walthers.com/exec/productinfo/770-3667